Fix phpunit 8 compatibility

Author: alexander-schranz

Test/KernelTestCase.php

@@ -208,6 +208,8 @@ protected static function createKernel(array $options = [])
     }
 
     /**
+     * @after
+     *
      * Shuts the kernel down if it was used in the test.
      */
     protected static function ensureKernelShutdown()
@@ -220,12 +222,4 @@ protected static function ensureKernelShutdown()
             }
         }
     }
-
-    /**
-     * Clean up Kernel usage in this test.
-     */
-    protected function tearDown()
-    {
-        static::ensureKernelShutdown();
-    }
 }